Image:Personnel in protective equipment cover an ambulance with a tarpaulin at Salisbury hospital

The comments come two days after countries across the world started expelling Russian diplomats in solidarity with Britain's view that the Russian state poisoned the pair with a Soviet-designed nerve agent called Novichok.

Montenegro, a former close Russian ally, is the latest country to expel a Russian diplomat.

A total of 25 other nations - including most in Europe, as well as Canada, the US and Australia - have jointly expelled more than 130 diplomats.

0:29

Video:Best friend: Show spy mercy - let him die

The Kremlin on Wednesday reiterated that Russia will "definitely retaliate" against the West over the expulsions.

Russia's ambassador to Australia, speaking a day after the country expelled him, said the world will enter into a "Cold War situation" if the West continues what he alleged is bias against Moscow.

Grigory Logvinov said: "The West must understand that the anti-Russian campaign has no future.

"If it continues, we will be deeply in a Cold War situation."

Washington has called on Russia to "accept responsibility for its actions and to demonstrate it is capable of living up to its international commitments".

"This depends on the American side, but the Russian side remains open," Dmitry Peskov said.

"Putin is ready and the Russian side is ready for mutually beneficial and mutually trustful relations with all countries including the US to the extent that our opponents or partners are ready for this."